1. Field of the Invention
This invention relates to a diffuser for a centrifugal compressor, and in particular, to a diffuser for a centrifugal compressor which can be operated highly efficiently over a wide range of flow rate, i.e. from a low rate to a rated one.
2. Description of the Prior Art
Generally, a centrifugal compressor includes a suction casing having a scroll, with an impeller being rotatably arranged in the suction casing for generating a high-speed air flow. Since the air flow going out of the impeller has large kinetic energy, a diffuser is provided on the downstream side of the impeller, i.e., outside the impeller, so that the kinetic energy of the flow discharged from the impeller may be converted into pressure energy. The diffuser is composed of stator blades radially arranged around the outer periphery of the impeller, and diffuser passages are formed between the stator blades.
In such a compressor, a phenomenon called surging, occurs in a low flow rate, where separation flows are generated on the suction or negative-pressure-side surfaces of the stator blades, and a sufficient pressure rise cannot consequently be attained.
In view of this, Japanese Patent Application Laid-Open No. 57-159998 discloses a diffuser which suppresses the surging even under a low-flow-rate condition. According to the disclosure, rotatable auxiliary or sub blades are provided in the inlet section of the diffuser, with the air flow through the diffuser being controlled by rotating these auxiliary blades, thereby preventing occurrence of separation flow on the suction or negative pressure-side surfaces of the stator blades.
However, while provision of the above-mentioned auxiliary blades helps to prevent the occurrence of surging under a low-flow-rate condition, another type of problem is encountered under a rated-flow-rate condition. Under the rated-flow-rate condition, the fluid lashes against these auxiliary blades, resulting in augmented pressure loss such as impact or collision loss and friction loss which leads to the operation efficiency of the compressor deteriorating.